Introduction to Bacterial Diseases Flashcards
What are microbes?
Microbes, or microorganisms, are typically considered to be any organism that is too small to be seen without a microscope.
They can be divided into three general classes: prokaryotes (i.e., bacteria and archaea) small eukaryotes (i.e., fungi, protozoa, algae, amoeba, and slime molds), and viruses.
What is the microbiota?
The healthy human body is home to trillions of microorganisms known as the microbiota (another term is normal flora).
These microorganisms reside in or on the body without causing disease.
The cohabitation of the body and the microbiota is an example of symbiosis.
In some cases, the body and the microbiota both benefit from this symbiosis, a type of relationship that is called mutualism.
In other cases, the symbiosis is beneficial only to the microorganisms. This type of relationship is called commensalism.
The human body is also constantly exposed to microorganisms in their environment through inhalation, ingestion, or physical introduction. What are the four potential outcomes from these encounters?
The microorganism may:
a) pass through the body as a harmless transient.
b) colonize the body and become part of the microbiota.
c) establish an infection in the body.
d) establish an infection in the body and cause disease.
Differentiate between infection and disease.
Although the terms infection and disease are frequently used interchangeably, they differ in meaning.
Infection refers to the invasion or colonization of the body by pathogenic (disease-causing) microorganisms, but this does not always result in damage or injury to the body.
Disease is damage or injury to the body that impairs function.
What influences the relationship between the host and the microorganism?
This relationship is influenced by:
a) the pathogenicity of the microbe, and
b) the resistance or susceptibility of the host. For example, the defence mechanisms of the host [both nonspecific (innate immune) and adaptive immune responses].
What are prokaryotes?
How do they compare with eukaryotes?
Prokaryotic cells are simple, single-celled organisms that lack a nucleus or any other membrane-bound organelles.
This is a big difference from eukaryotic cells, which have a membrane-bound nucleus and other internal membrane-bound organelles such as the endoplasmic reticulum, mitochondria, and lysosome/vacuole.
However, like eukaryotes, bacteria do have a cytoplasm which harbors all the critical components to live, such as DNA, RNA, proteins, ribosomes, etc., as well as a nucleoid composed of the chromosomal DNA and some proteins and other molecules.
Similar to the eukaryotic plasma membrane, all bacteria have a membrane surrounding the cytoplasm, which in bacteria is called the ‘cytoplasmic membrane’ (denoted in Figure 1 as a ‘cell membrane’).
Another structural difference between eukaryotic and prokaryotic cell is that eukaryotic cells such as human cells are ~10-100 times larger than bacteria.
What is the cytoplasmic membrane?
This is a phospholipid bilayer that keeps all of the internal components inside the cell and also contains proteins that perform various functions such as transport and some enzymatic activities.
Describe the cell wall that bacteria have.
However, unlike human cells, bacteria also have a protective cell wall outside the cytoplasmic membrane (together the cell wall and cytoplasmic membrane are often referred to as the ‘cell envelope’).
The cell wall of all bacteria contains a covalently bonded layer composed of a polymeric molecule called peptidoglycan (PG) outside the cytoplasmic membrane.
PG is unique to bacteria and consists of a covalently-linked glycan backbone with peptide tails, which are also covalently cross-linked together.
Because of its covalent bonds, PG acts as a protective mesh, allowing the bacteria to survive hypo-osmotic stress.
What additional components may bacterial cell walls contain?
What are their functions?
Cell walls can also contain additional components such as: flagella, which provide the bacteria with swimming motility; pili, which are hair-like structures primarily used in adherence to surfaces; and capsules, which can also mediate attachment to surfaces but also confer protection from predatory cells such as phagocytes.
What is the difference between Gram-negative and Gram-positive bacteria?
Gram-negative bacteria also have an outer membrane outside the PG layer, while Gram-positive bacteria do not.
The PG of Gram-positive cells is quite thick, while Gramnegative cells have thinner peptidoglycan between the two membranes.
The outer membrane of Gram-negative bacteria also contains a large amount of lipo-polysaccharide (LPS).
The lipid portion of LPS is intercalated with the phospholipids, while the saccharide portion sticks out into the environment.
LPS plays an important role in the host-pathogen interaction.
The peptidoglycan of Gram-positive bacteria can also contain glycopolymers called teichoic acids (TA) and lipo-teichoic acids (LTA).
These structures are important for several aspects of bacterial physiology such as shape determination and cell division regulation.
Briefly describe the Cytoplasmic components of bacteria: DNA, RNA, protein, and machinery catalyzing DNA replication, transcription (RNA polymerase), and protein synthesis (ribosomes).
Bacteria contain and synthesize all of the necessary cellular components to be free-living.
Like eukaryotes, bacteria transcribe DNA into RNA, translate RNA into protein, and replicate their own DNA.
However, there are several significant differences that are not only important to take into account regarding general bacterial physiology and function, but also with respect to therapy measures.
Compare DNA structure and replication of bacteria and eukaryotes.
Cells of higher eukaryotes like humans harbor their DNA on multiple linear chromosomes wrapped around histone proteins, each of which replicates independently.
In contrast, bacteria typically have circular chromosomes, usually only one, no histones, and only one origin of replication per chromosome.
Bacteria can also contain additional smaller circular DNAs called plasmids, which have their own origins of replication and thus replicate independent of the chromosome.
Bacteria also do not contain introns and exons, and, unlike the one-promoter-one-gene structure of eukaryotes, bacteria typically harbor their genes in ‘operons’ – i.e., one promoter can control transcription of many genes.
Replication of bacterial DNA is also ~200x faster than that of eukaryotic DNA.
DNA polymerases are different too, as are accessory proteins like DNA gyrase, which relieves stress during unwinding by DNA helicase.
DNA gyrase is much more important for bacterial DNA replication than for eukaryotic DNA replication.
Compare RNA structure and synthesis of bacteria and eukaryotes.
Bacterial DNA exists in operons, thus one messenger RNA (mRNA) can be translated into multiple proteins.
Prokaryotic mRNA lacks most of the modifications found on eukaryotic mRNA, its lifespan is shorter, and because everything occurs in the cytoplasm (vs. eukaryotes, which must transport RNA from the nucleus to the cytoplasm), transcription and translation in bacteria occur nearly simultaneously.
Bacterial mRNA lifespan is also much shorter than that of eukaryotic mRNAs.
The makeup of full RNA polymerase – i.e., the key subunits and accessory factors – is also different.
For instance, bacteria use a small number of sigma factors to guide the RNAP holoenzyme to promoter sites while eukaryotes have many different transcription factors for this purpose.
Compare protein synthesis of bacteria and eukaryotes.
Both prokaryotes and eukaryotes use ribosomes composed of ribosomal RNA (rRNA) and protein to synthesize new proteins from mRNA.
However, the sizes and several structural aspects of the ribosomes are different.
Bacterial ribosomes are smaller than those of eukaryotes.
Bacteria ribosomes are ‘70S’, whereas eukaryotic ribosomes are ‘80S’.
The ‘S’ stands for Svedburg unit and relates to the density of the molecules and how fast they sediment – i.e., during centrifugation.
Both bacterial and eukaryotic ribosomes are composed of a large and a small subunit: bacterial subunits are 50S and 30S; eukaryotic submits are 60S and 40S.
What is the reason antibiotics targeting the bacterial ribosome do not affect eukaryotic ribosomes?
An important feature of the bacterial 30S subunit is one of its individual components, the 16S rRNA.
This rRNA differs significantly from the human 40S subunit 18S rRNA, and these differences are the primary reason why antibiotics targeting the bacterial ribosome do not affect those of eukaryotes (except mitochondrial ribosomes which are similar to those of bacteria, but they are significantly protected by being inside the membrane-bound organelle at antibiotic doses typically used).
What is virulence?
Virulence is a measure of the pathogenicity of a microorganism, i.e., it is a quantitative concept.
Virulence can be expressed as the cell number that will elicit a pathogenic response in the host within a given time period.
For example, the infection with a relative small number of a highly virulent bacterium (Streptococcus pneumoniae) will kill a certain number of mice, whereas an infection with 104 –X more of a less virulent bacterium (Salmonella typhimurium) is required to kill the same number of mice.
